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Probing Ionic Complexes of Ethylene and Acetylene with Vacuum-Ultraviolet Radiation
journal contribution
posted on 2016-03-16, 00:00 authored by Biswajit Bandyopadhyay, Tamar Stein, Yigang Fang, Oleg Kostko, Alec White, Martin Head-Gordon, Musahid AhmedMixed
complexes of acetylene–ethylene are studied using
vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical
calculations. These complexes are produced and ionized at different
distances from the exit of a continuous nozzle followed by reflectron
time-of-flight mass spectrometry detection. Acetylene, with a higher
ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning
the VUV energy and initializing reactions either from a C2H2+ or a C2H4+ cation. Pure acetylene and ethylene expansions are separately carried
out to compare, contrast, and hence identify products from the mixed
expansion: these are C3H3+ (m/z = 39), C4H5+ (m/z = 53), and C5H5+ (m/z =
65). Intensity distributions of C2H2, C2H4, their dimers and reactions products are plotted
as a function of ionization distance. These distributions suggest
that association mechanisms play a crucial role in product formation
closer to the nozzle. Photoionization efficiency (PIE) curves of the
mixed complexes demonstrate rising edges closer to both ethylene and
acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ)
to study the structures of the neutral and ionized dimers, calculate
their adiabatic and vertical ionization energies, as well as the energetics
of different isomers on the potential energy surface (PES). Upon ionization,
vibrationally excited clusters can use the extra energy to access
different isomers on the PES. At farther ionization distances from
the nozzle, where the number densities are lower, unimolecular decay
is expected to be the dominant mechanism. We discuss the possible
decay pathways from the different isomers on the PES and examine the
ones that are energetically accessible.
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ionization distancenumber densitiesreactions productsproduct formationethylene expansionsenergy surfaceC 5 H 5use densityionization energiesC 2 H 4C 2 H 2photoionization mass spectrometryPESPure acetyleneinitializing reactionsC 3 H 397Xionization distancesnozzleassociation mechanismsIonic Complexesunimolecular decayVUV energyacetylene ionization energiescomplexisomersdecay pathwaysPhotoionization efficiency
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